Traditionally, civil engineers base design guidelines on elastic section modulus. If you see a section modulus number cited without reference to whether it is plastic or elastic, then the general rule is that this is elastic section modulus. In the rare instances when plastic section modulus is used, the word “plastic” almost always explicitly precedes section modulus. Plastic section modulus is usually used to calculate according to the plastic method and to determine the plastic capacity of cross-sections. The elastic and plastic resistance values are different, and the calculation results may be different.